In the recent drive for higher integration and operating speeds in LSI devices, the pattern rule is made drastically finer. Active efforts have been made on the development of micro-processing technology using deep ultraviolet lithography and vacuum ultraviolet lithography. In particular, the ArF excimer laser as a light source of the generation next to the KrF excimer laser is recognized essential to the advanced semiconductor photolithography of 90 to 45-nm node.
One of resist material characteristics required for the photolithography using ArF excimer laser as the light source is a transmittance at wavelength 193 nm. Transparency is indispensable to achieve a high sensitivity and high resolution. For example, the base resins proposed thus far include poly(meth)acrylic acids and derivatives thereof having tertiary ester type acid-labile protective groups as typified by 2-ethyladamantyl and 2-methyladamantyl. They ensure a certain level of etch resistance attributable to a high carbon density of adamantyl group and the transparency of a single resin, achieving some advantages including a high sensitivity and a certain level of resolution. See JP-A 9-73173 and JP-A 9-90637.
However, since acid labile groups as typified by 2-alkyl-adamantyl have high rigidity and hydrophobicity in themselves, polymers having a large proportion of such compounds incorporated therein become very rigid and hydrophobic overall. This causes swelling of a resist film during development, leading to pattern fusion or collapse and pattern sidewall roughening, known as line edge roughness (LER). These polymers are thus impractical in the lithography because a further pattern rule reduction is needed in the future.
Recently proposed were polymers comprising as constituent units (2-adamantyloxy)-methyl methacrylate, 3-hydroxyadamantyl methacrylate, and a lactone structure-bearing methacrylate such as γ-butyrolactone methacrylate or norbornane lactone methacrylate (see WO 2005080473). They have a good sensitivity even when a weak acid is generated from the acid generator, succeeding in acquiring relatively good resist characteristics.
However, resins comprising norbornane lactone methacrylate have an increased LER due to swelling. Resins comprising γ-butyrolactone methacrylate have an increased acid diffusion and an increased size difference between isolated and grouped patterns (known as I/G bias). These resins are thus impractical in the lithography because a further pattern rule reduction is needed in the future.
It is confirmed that polymers comprising as constituent units adamantylmethoxymethyl methacrylate, 3,3′-dihydroxyadamantyl methacrylate, and a lactone structure-bearing methacrylate such as oxanorbornane lactone methacrylate or 9-methoxycarbonyl-4-oxatricyclo[4.2.1.03,7]nonan-5-on-2-yl methacrylate are certainly superior in process margin and exposure margin (see JP-A 2005-234449). However, since the effects for 3,3′-dihydroxyadamantyl methacrylate to impart certain rigidity to resin and restrain acid diffusion are offset by the effect for adamantylmethoxymethyl methacrylate to promote acid diffusion due to its side chain flexibility, it is difficult to suppress the I/G bias. These resins are thus impractical.
It is also confirmed that polymers comprising (1-adamantyloxy)methyl methacrylate and oxanorbornane lactone methacrylate are certainly superior in process margin and exposure margin (see JP-A 2005-234450). Since they lack 3-hydroxyadamantyl methacrylate and 3,3′-dihydroxyadamantyl methacrylate as found in the foregoing polymers, they fail to hold down acid diffusion by introducing a hydroxyadamantane structure as typified by 3-hydroxyadamantyl methacrylate and 3,3′-dihydroxyadamantyl methacrylate for imparting a certain rigidity to resins. In addition, these resins have a lower carbon density and hence, lower etch resistance and are thus impractical.
In summary, not only resolution and etch resistance are required, but it is also required to restrain line edge roughness (LER) and size difference between isolated and grouped patterns (I/G bias). The above problems become serious particularly at sizes less than the KrF exposure wavelength of 248 nm. Holding down acid diffusion is an effective method for reducing the I/G bias. However, minimizing acid diffusion gives rise to problems including the asperity of pattern sidewalls by standing waves and an increased LER. It is thus a difficult task to find a compromise between LER and I/G bias improvements. Enhancing the contrast of exposure dose dependence of a dissolution rate of resist film in developer (i.e., dissolution contrast) is effective not only for resolution, but also for LER improvement. In order to meet both minimal LER and minimal I/G bias while maintaining a high resolution, it is desired to have a resist material having a reduced acid diffusion length and a high dissolution contrast.